This invention relates to an improved photographic element which exhibits improved light sensitivity and speed and to a process for recording images therewith. In a specific aspect, this invention is directed to an improved photographic element and to a process for its use in which a visible image is selectively produced by the interaction of an improved photoreductant with a dye precursor in image areas, while the background areas remain stable and of low optical density in the presence of actinic radiation and without removal or alteration of the dye precursor.
It is well known in the photographic arts to record images by incorporating within a radiation-sensitive layer of a photographic element a dye precursor of low optical density capable of conversion to a visible dye. In order to avoid dye printout in background areas after exposure it is conventional practice to inactivate and/or wash out the dye precursor. Where the dye is formed by oxidation of its precursor there is frequently a problem with background printout attributable to atmospheric oxidation of the dye precursor remaining in the background areas. The loss of contrast is, of course, further accelerated if the dye itself also tends to fade.
The reduction of a tetrazolium salt to form a formazan dye image is generally well known in the art. For example, in Brault et al U.S. Pat. No. 3,642,478, issued Feb. 15, 1972; Brault et al U.S. Pat. No. 3,655,382, issued Apr. 11, 1972 and Bissonette et al U.S. Pat. No. 3,671,244, issued June 20, 1972, there are disclosed processes for utilizing a zero valent metal image to reduce a tetrazolium salt and produce a formazan dye image.
A system that does not require the presence of a metal image is disclosed by Jaeken et al in British Pat. No. 670,883 published Apr. 30, 1952. Jaeken et al incorporates into a photographic element a tetrazolium salt and a reducing agent precursor which on exposure to light produce a formazan dye image. After exposure the photographic element is washed in up to three successive aqueous baths intended to fix the image by removal of the unreduced tetrazolium salt remaining in the background areas, enhance the image density and remove the remaining reducing agent precursor and the reaction products which, if left in place, would stain the background of the photographic element. Reducing agent precursors such as ferric salts, vanadic compounds, tungstic compounds and uranium salts are employed. Baths containing organic hydroxy acids or their ammonium salts are used to remove the brown oxides produced by ferric salts. Aqueous ammonia solutions are optionally used for image enhancement while water or dilute acid fixing baths are used to remove residual tetrazolium salts.
As improvements on the teachings of Jaeken et al, Telefunken British Pat. No . 1,016,822, published Jan. 12, 1966, and Schiele U.S. Pat. No. 3,278,366, issued Oct. 11, 1966, teach the reduction of a tetrazolium salt to a formazan dye in response to radiation of 200 nm or less. In contrast to the system of Jaeken et al in which the reducing agent precursor is acted on by radiation to produce a reducing agent, Telefunken and Schiele activate the tetrazolium salt by radiation to cause it to react with the available reducing agent. For very high-energy radiation below 200 nm, no separate reducing agent is required to convert the tetrazolium salt to a formazan dye. The exposed photographic elements of Schiele and Telefunken can be viewed without fixing since they are insensitive to radiation within the visible spectrum. Of course, re-exposure to actinic radiation would result in background printout.
Other imaging systems have produced a silver image by photoinitiating a reduction of silver halide or silver salt to the silver. The reductant, or progenitor of the reductant has been in some cases a quinone. However, the specific quinones of those systems were not easily photoconverted, due to the lack of internal hydrogen sources as defined hereafter. The unpredictability of the effectiveness of structurally related compounds is also a recognized feature of these prior systems. Examples of individual quinones for such systems are disclosed in U.S. Pat. Nos. 3,287,129; 3,409,438; and 3,529,963. U.S. Pat. No. 3,287,129, while suggesting a broad class of quinones including those having an amino group attached to the naphthoquinone ring, does not recognize the superiority specifically available with internal hydrogen source quinones. Thus, the examples set forth in detail all lack internal hydrogen sources.
Still other imaging systems have incorporated quinones, some of which are internal hydrogen sources, to reduce portions of the imaging systems. However, these have been limited to the use of the quinones as photoreductants for the sole purpose of deactivating the imaging system to prevent printout. Examples are disclosed in U.S. Pat. No. 2,322,982, wherein the imaging means are diazonium salts and couplers, and U.S. Pat. Nos. 3,383,212; 3,390,994; and 3,390,996 wherein the quinone is converted to a reducing agent to deactivate a photo-oxidant in a leuco dye system.